1. Field of the Invention
The present invention relates to a process for minimizing disintegration in the blast furnace and low shaft furnace of iron ores and iron ore agglomerates.
2. Brief Description of the Background of the Invention Including Prior Art
The disintegration of iron ores and iron ore agglomerates at the beginning of reduction (reduction disintegration) in the low temperature range from about 400 degrees centigrade to 600 degress centigrade in a weakly reducing atmosphere is regarded by competent professionals as an extremely negative characteristic of a blast furnace burden material. As is well known, high reduction disintegration results in high flue dust production and scaffolding, hinders gas distribution, raises the consumption of fuel and reducing gas, lowers the production capacity and deteriorates the pig iron quality. The technical drawbacks listed above mean, at the same time, quite remarkable economic disadvantages. For instance, high reduction disintegration can bring about a rise in fuel consumption--either in the form of coke or oil--by up to 10 percent of normal consumption, which presently amounts to between 400 and 500 kg per ton of pig iron.
An early teaching exists with the purpose to prevent the decomposition of iron ores and iron ore briquettes, which is contained in the German Patent DE-P289,727 of Jan. 14, 1916 to Det Norske. According to the teaching of this reference a glaze-like coating is formed with sodium chloride around the ore particles. According to the teaching of the reference the treatment materials are to be added to the ore or ore products during the smelting process in a furnace. It is also mentioned that a certain amount of the treatment salt is added to the ore or iron ore briquettes immediately ahead of its use in the blast furnace with the goal of a glaze formation. Thus the reference teaches the formation of a glaze in order to enhance the strength via the glazing. The reference teaches further that based on this step the catalyst effectiveness of the ore is to be reduced, which enhances the deposition of carbon in the ore or in the briquettes, a clearly undesirable feature.
Other attempts of strengthening ores or sintered products have been made such as are taught for example in D. F. Ball et al. "Agglomeration of Iron Ores", American Elsevier Publishing Co., New York, N.Y. (page 304, lines 27 to 32); A Be in 677,083 of Aug. 25, 1966 (C.N.R.M.); and K Meyer, "Pelletizing of Iron Ores", Springer Verlag, Berlin, 1980, pages 37, 56, 109, 116, 126, 161, and 184 to 188. According to these references the addition of reagents is provided before their production, that is the additives are added already to the starting materials for making the agglomerates. This method entails the disadvantage that the additives can be directed to the production process of the agglomerates, but they will in general not be specific for the specific purpose of controlling the behavior of the agglomerated material in the blast furnace.
In order to keep disintegration of the blast furnace burden as low as possible, cerain requirements must be met by the major burden materials--sinter and pellets--which are permanently controlled and which must be corrected if the limiting values fall below specifications.
Such corrections are made, for instance, by changing basicities, the type of base-bearing components, the degrees of oxidation or by operational steps which always mean higher fuel consumption which can exceed the normal amount by 5 to 10%.
Nevertheless, in spite of all these measures, today's requirements relating to reduction disintegration, cannot be met. Besides, there are certain pellet grades whose tendency to disintegration at the beginning of reduction in the blast furnace is quite pronounced after a storage period from 2 to 3 weeks only.
Even though the disintegration which is caused by the storage, can be met either by shorter storage periods or by adding certain amounts of basic additives, it must be pointed out that this can be realized to a limited extent only, with higher production cost of the pellets and, in part, metallurgical disadvantages. Often there is the only choice: to directly adapt the production to the actual needs. As a consequence, operating plants must frequently close down.
Here, the invention proves a remedy. It is the objective of this invention to propose a method, by which the disintegration of iron ores and iron ore agglomerates, which are charged as burden into the blast furnace, and which disintegration starts at the beginning of reduction in the low-temperature range, is minimized. The invention solves the problem in such a manner that iron ore agglomerates, after their induration and prior to smelting, and iron ores prior to smelting in the blast furnace, are treated with reagents which prevent them, to a large extent, from disintegrating during reduction in the temperature range from 400.degree. C. to 600.degree. C.
According to the invention, reduction disintegration of a burden component can decisively be minimized, if the burden component, prior to its charge into the blast furnace, is treated with halogen solutions. These solutions or suspensions of halogens and their compounds, respectively, may consist of NaCl, CaCl.sub.2, KCl, KJ, KBr, Na.sub.2 SiF.sub.6. In their simplest form the halogens may, for instance, be brought in from sea water (NaCl).
The advantages of the invention are, that the treatment is done in a very simple manner by powdering or spraying on or by immersion of the burden material into the corresponding solutions which are effective even in high dilution. The result of the treatment is a considerable minimizing in disintegration, depending on the type of burden, the reagent and its concentration. Other quality characteristics, as, for instance, mechanical strength, reducibility, strength at 1,050.degree. C. reduction temperature are not at all changed by the treatment.
The invention method for minimizing disintegration of pieces of iron ores and iron ore agglomerates, preferably sinter material and pellet form in a reduction furnace comprises treating iron ores and iron ore agglomerates otherwise ready to be fed to the reduction furnace with reagents, which minimize disintegration of the ores and agglomerates when exposed to temperatures in the range of from about 400 to 600 degrees centigrade.
The reagents employed can be solutions containing halogenides, a member of the group comprising sea water, liquids similar to sea water, and aqueous solutions containing halogen in an amount of from about 0.1 to 50 grams per liter, and preferably aqueous solutions containing halogenides, where the amount of halogen is from about 1 to 20 gram per liter of solution, which appears for technical reasons presently to be the most preferred method based on technical practicability. Materials useful in providing the halogenide to the pieces include the halogenides of the metals having a number of less then about 55 and preferably of less than 30 in the periodic table of Mendelejev. Preferred halogenides are those which evaporate at temperatures of from about 600 to 800 degrees centigrade under blast furnace conditions. The specific reagent employed can be an aqueous solution containing a member of the group consisting of sodium chloride, magnesium chloride, calcium chloride, potassium chloride, potassium iodide, potassium bromide, sodium hexafluoro silicate, and mixtures thereof. The treating can be performed until the pieces have an analytical content of from about 0.001 to 0.1 weight percent in halogen and preferably of from about 0.01 to 0.03 weight percent in halogen.
It has further been found that higher rates of addition as compared to the rates recited according to the present invention do not enhance the effects of the present invention. In contrast, higher rates delay the reduction too much and they load the blast furnace with damaging elements extensively and unnecessarily. In addition, it was found in tests at 500 degrees centigrade that there was somehow an increased decomposition if larger amounts of halogenides are present.
It has been found that a critical point as to the strength of cohesion of pellets, sinter, sintered briquettes and ores under reducing conditions is the reduction in the temperature region of from about 400 to 600 degrees centigrade in a weakly reducing atmosphere, as it occurs in the upper parts of a blast furnace. A loss in strength is expressed as a so-called falling apart of the grains and pieces. An increased and too high falling apart of the the grains results in an interference with the gas flow passage, in an increased flue dust from the throat of the furnace, and finally in a lower smelting efficiency. It has been found that a suitable test method for testing the grain and piece disintegration under reducing conditions is a externally heated electrical rotary tube furnace or a rotary kiln. After the testing in such a furnace the sample is subjected to a sieve analysis, since the grain structure of the reduced sample is a measure for the grain and piece disintegration. The percentage weight part of the tested sample not passing a sieve of 6.3 millimeter or, respectively 3.15 millimeter diameter of the apertures (+6.3 mm or +3.15) is designated as disintegration strength and the weight percentage of material passing through a sieve with 0.5 millimeter apertures (-0.5 millimeter) is designated as disintegration abrasion dust. For example, in the Federal Republic of Germany at present the following limiting values are to be achieved for the use of pellets and ores in modern blast furnaces:
At least 80 weight percent of grains and pieces with a size of more than 6.3 millimeter. PA0 At most 15 weight percent of grains and pieces with a size of less than 0.5 millimeter.
In theory, according to the present invention all compounds can be considered for the treatment which contain halogenides. The effect of such a treatment however is the larger, the larger the contents of halogenides is in a compound employed, since only the halogenides are responsible for an effect. The reason for this is that the halogen compounds introduced by dipping, spraying or mingling with powder followed by treatment with water results in a deposition of the halogen compounds at the grains or pieces and preferably at cracks or pores. This reduces or prevents the flow and passage of the reducing gases as long as halogenides are still present. The halogenides evaporate if their partial vapor pressure becomes sufficiently high. This depends on the temperature level and the kind of the halogenide carrier, since the evaporation temperature is in the area of from about 600 to 800 degrees depending on the kind of halogenide carrier. Only above the temperature range of about 600 to 800 degrees centigrade the surfaces, cracks and pores become free again and only thereafter the reducing gases become substantially active. This process can be controlled with the kind of addition and the rate of addition of the additives. The premature reduction as a substantial cause for the disintegration can thereby be controlled and/or prevented. The reduction decomposition can be reduced up to about zero depending on the rate and kind of the additive. Already in case of addition rates from about 0.001, which is equal to 10 grams per ton of pieces of agglomerate or ore, the effect is observed. In most cases rates of about 0.02, which is equal to about 200 grams per ton, have shown to be sufficient in all cases as nearly completely preventing reduction disintegration. The halogenides can be added at the producer of the pieces or at the user location. In addition, agglomerates aged by storage can again be made free of disintegration.
The reagents can be sprayed upon the pieces. This can be effected in apparatus like that employed in a coke plant for cooling the coke. Alternatively, the reagents contain halogen and are in solid form at the time of treatment. The pieces can also be immersed in a bath containing a solution of halogenides. This can for example be effected with a dipping bath. The pieces can be disposed on a transporting belt during the treatment process. The times of treatment are not critical, but a relatively homegeneous distribution of the reagent over the pieces is preferred.
Furthermore, the present invention provides pieces of iron ore and iron ore agglomerates containing a deposit of a soluble halogenide on its surfaces accessible to contact with liquid, where the total amount of halogenide corresponds to an analytical content of from about 0.01 to 0.05 weight percent of halogen of the piece. The halogenide can comprise a member of the group consisting of sodium chloride, magnesium chloride, calcium chloride, ferrous chloride, ferric chloride, potassium chloride, manganese chloride, and mixtures thereof and the analytical halogen concentration in the pieces can be from about 0.001 to 0.1 and is preferably from about 0.01 to 0.03 weight percent.
Thus the invention method minimizes disintegration in the blast and low-shaft furnace, respectively, of iron ores and iron ore agglomerates at the beginning of reduction in the temperature range between 400 and 600 degrees centigrade, where the iron ores before charging into smelting aggregates, and the iron ore agglomerates after their production and before smelting are treated with reagents, which, to a great extent, inhibit or, respectively, minimize disintegration.
The reagents can be in the form of solutions or sea water or liquids similar to sea water, which contain halogens in the order of 0.1 gram per liter to 50 gram per liter. The solutions containing halogens can be sprayed upon the iron ores and iron ore agglomerates, respectively. The reagents intended for treatment and containing halogens, can be of a solid form. The iron ores and iron ore agglomerates, respectively, for halogen absorption are immersed in a bath which consists of a solution containing halogens. The reagents existing in solid form, can be applied to iron ores and iron ore agglomerates, respectively. The iron ores and iron ore agglomerates, respectively, which were treated with the halogen compounds, may be protected against washing out by water. The treatment of the iron ores and, respectively, the iron ore agglomerates either with solutions containing halogens, or, otherwise, in solid form, with reagents containing halogens, is effected immediately prior to charging into smelting aggregates.
However, special attention should be paid to the fact, that it is possible to produce deliberately, through a reduced fuel rate, agglomerates which still meet the requirements of mechanical strength, but whose disintegration values are not satisfactory. By treating these agglomerates in accordance with the present invention, their reduction strength is raised to the required values. In this way, it is possible to save energy already in agglomeration.
The detrimental elements resulting from such treatment, which may enter the blast furnace, together with the burden, are discharged either with the slag or in the top gas.